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Anaerobic Digestion

Anaerobic digestion is a biological process that produces a gas principally composed of methane (CH4) and carbon dioxide (CO2) otherwise known as biogas.
These gases are produced from organic wastes such as livestock manure, food processing waste, etc.

Anaerobic processes could either occur naturally or in a controlled environment such as a biogas plant.
Organic waste such as livestock manure and various types of bacteria are put in an airtight container called digester so the process could occur.
The picture to the right below shows a biogas digester at a dairy farm in Cottonwood, California.
Depending on the waste feedstock and the system design, biogas is typically 55 to 75 percent pure methane.
State-of-the-art systems report producing biogas that is more than 95 percent pure methane.

Types of Biomass Power Plants

The first step is the decomposition (hydrolysis) of plant or animal matter. This step breaks down the organic material to usable-sized molecules such as sugar.

The second step is the conversion of decomposed matter to organic acids.

Finally, the acids are converted to methane gas.

Process temperature affects the rate of digestion and should be maintained in the mesophillic range (95 to 105 degrees Fahrenheit) with an optimum of 100 degrees F.
It is possible to operate in the thermophillic range (135 to 145 degrees F), but the digestion process is subject to upset if not closely monitored.

Many anaerobic digestion technologies are commercially available and have been demonstrated for use with agricultural wastes and for treating municipal and industrial wastewater.

At Royal Farms No. 1 in Tulare, California, hog manure is slurried and sent to a Hypalon-covered lagoon for biogas generation.
The collected biogas fuels a 70 kilowatt (kW) engine-generator and a 100 kW engine-generator.
The electricity generated on the farm is able to meet monthly electric and heat energy demand.

Given the success of this project, three other swine farms (Sharp Ranch, Fresno, and Prison Farm) have also installed floating covers on lagoons.
The Knudsen and Sons project in Chico, California, treated wastewater which contained organic matter from fruit crushing and wash down in a covered and lined lagoon.
The biogas produce is burned in a boiler.
And at Langerwerf Dairy in Durham, California, cow manure is scraped and fed into a plug flow digester.
The biogas produced is used to fire an 85 kW gas engine.
The engine operates at 35 kW capacity level and drives a generator to produce electricity.
Electricity and heat generated is able to offset all dairy energy demand.
The system has been in operation since 1982.

Most anaerobic digestion technologies are commercially available.
Where unprocessed wastes cause odor and water pollution such as in large dairies, anaerobic digestion reduces the odor and liquid waste disposal problems and produces a biogas fuel that can be used for process heating and/or electricity generation.